A general aviation pilot reported an NMAC with a corporate jet descending on a visual approach to a nearby airport.

Narrative

I was returning to ZZZ from the North East. I had planned to intercept the ZZZ RNAV XXL approach course at 4;500 ft MSL and about 9 miles from ZZZ. This would put me in ZZZ Tower airspace for my initial call for landing. Since the approach course is very close to the Northern extent of ZZZ1 class D airspace I was at 4;500 to ensure lateral and vertical separation from ZZZ1 Delta. I have used this routing to approach ZZZ for landing many times when arriving from the North East.I have dual traffic displays in my aircraft. One is provided by the Garmin GDU 465 display and the other by my Garmin GPS 175 IFR GPS. I was also monitoring ZZZ approach on XXX.X. When I was about 7 nm North of ZZZ1 ZZZ approach called my callsign to ask if I was on frequency and I replied. I was asked if I was landing ZZZ1 and replied that I was landing at ZZZ. He advised me he had a Citation on right base to ZZZ1 XY and that I should descend. I replied that I needed to maintain 4;500 to ensure clearance from ZZZ1 Delta. Approach insisted that I descend for ZZZ but gave me no clearance altitude or heading. I turned right and descended then acquired the Citation visually. It passed almost directly above me. Later review of ADS-B data showed the vertical separation was 450 ft.My analysis of the ADS-B data replay showed that; if I had followed my intended path maintaining 4.500; the Citation would have passed under me with 575 ft separation. Since my aircraft is high wing I would have acquired it visually sooner.In this case the controller's actions reduced the separation from what it would have been had he taken no action. This event makes me very reluctant to reply to a controller when I'm monitoring. Had I not replied the controller could have given the Citation a 5-degree left turn and ensured lateral and vertical separation. I was VFR in class E airspace and had not requested ATC services.